Automotive exhaust mufflers function to attenuate the noise associated with the flow of exhaust gas from the engine. In the typical design process, automotive engineers will assess the flow characteristics of the exhaust gas and the space available on the vehicle for the muffler. The engineers then will use known algorithms to develop an appropriate pattern of tubes and chambers within the muffler for attenuating the noise of the exhaust gas. However, the pressure of the flowing exhaust gas can cause vibrations in the walls of the muffler. These vibrations can cause a "shell ring"noise independent of the noise associated with the flowing of the exhaust gas. Thus the muffler often must be designed to dampen the "shell ring".
The typical prior art muffler employs a plurality of tubes supported in a parallel array by a plurality of transversely extending, identically configured oval or circular baffles. The tubes and baffles are then disposed within a generally tubular outer shell of circular or oval cross-sectional configuration conforming to the shape of the baffles. To prevent the outer shell from vibrating excessively and generating a "shell ring"the typical prior art muffler further includes an outer wrapper which is wrapped around the tubular outer shell. The outer wrapper tends to dampen the vibrations of the shell and thereby avoid the above described shell ring. The prior art muffler then is completed by affixing end caps or headers to the opposed longitudinal ends of the muffler.
The prior art further includes mufflers formed partly or entirely from stamped components. For example, an array of tubes may be disposed within an outer shell defined by a pair of opposed stamp formed sheets of metal. The sheets of metal in these prior art mufflers may be stamped to include a generally planar peripheral flange with one or more chambers formed to extend out of the plane defined by the peripheral flange. The opposed external shells of these prior art mufflers are assembled around the array of tubes such that the tubes are disposed within the chambers defined by the external shells. Some such prior art mufflers employ separate conventional tubes disposed within the stamped external shell. However, other prior art mufflers employ tubes which also are defined by stamped components. In particular, a pair of internal plates may be stamped to define channels therein. The channels are disposed to define an array of tubes when the internal plates are secured in face-to-face relationship with one another. The stamp formed internal plates may then be disposed between the above described stamp formed external shells to define the muffler.
Mufflers employing stamp formed external shells are subject to the same potential for shell ring as the above described prior art mufflers with tubular outer shells. Various attempts have been made to eliminate or minimize the shell ring in stamped mufflers by forming arrays of grooves or ridges in the external shells. U.S. Pat. No. 2,484,827 issued to Harley on Oct. 18, 1949 and shows a muffler with deep corrugations formed in each of two opposed casing members. More particularly, each casing includes a peripheral flange with corrugations disposed intermediate the associated peripheral flange. The bottom of each corrugation is disposed to lie within the plane of the peripheral flange, with the top of the corrugations extending out of the plane of the peripheral flange. The corrugations of one casing are disposed to be angularly aligned to the corrugations of the opposed casing. The casings of the muffler shown in U.S. Pat. No. 2,484,827 are mounted together such that the peripheral flanges are in abutting contact with one another and such that the bases of the respective ridges of one casing substantially contact the bases of the ridges in the opposed casing at the points of intersection. The corrugations are provided to define a complex chamber construction through which exhaust gas may flow and not to dampen shell ring. This construction would necessarily require very complex draws in the metal of the casings and would substantially preclude the use of tubes and chambers in the muffler to attenuate a specified pattern of exhaust gas noise.
Another prior art muffler with grooves in an external shell is shown in U.S. Pat. No. 3,176,791 which issued to Betts et al. on Apr. 6, 1965. One external shell of the muffler shown in U.S. Pat. No. 3,176,791 includes an array of very deep parallel grooves, the bottoms of which lie almost in the plane of the peripheral flanges. The opposed external shell is provided with shallower grooves which are generally circular in cross-section and which extend from a first location spaced from the peripheral flange to an opposed location on the muffler also spaced from the peripheral flange. Although reinforcements of this type may reduce vibrations within the walls of a chamber somewhat, it has been found that the entire chamber may vibrate relative to the substantially more rigid peripheral flange. Thus, mufflers with reinforcing grooves or ridges that begin and/or terminate at locations spaced from a peripheral flange have been found to be only marginally effective in reducing shell ring.
The Assignee of the subject invention has made several very significant improvements in stamped mufflers. One of these improvements is shown in U.S. Pat. No. 4,924,968 which is directed to a reinforced outer shell for a stamp formed muffler. In particular, the shell of the muffler shown in U.S. Pat. No. 4,924,968 includes a generally parallel array of reinforcing grooves. Each reinforcing groove intersects the peripheral flange of the external shell at two opposed locations thereon. The extension of each reinforcing groove the entire distance to the associate peripheral flange helps to both reinforce the walls of the external shell while further ensuring a rigid interface of the entire external shell relative to the peripheral flange. The disclosure of U.S. Pat. No. 4,924,968 is incorporated herein by reference.
A further improvement in stamped muffler technology is shown in copending U.S. patent application Ser. No. 577,495 for "STAMP FORMED MUFFLER WITH LOW BACK PRESSURE" which was filed by Michael W. Clegg et al. on Sep. 4, 1990 and which is assigned to the Assignee of the subject invention. Certain embodiments of the muffler shown and described in pending application Ser. No. 577,495 include an external shell defining a single large chamber. Certain embodiments further show and describe mufflers with internal plates defining fairly large in-line expansion chambers or cans. It has been found that in many of these embodiments the forces exerted by the exhaust gas flowing through the muffler will create vibrations and/or dimensional changes in the internally disposed in-line expansion chambers of the muffler. Vibrations of an internal chamber may cause shell ring similar to that described above with respect to chambers defined by the external shell. Furthermore, dimensional changes of the internally disposed in-line expansion chambers can alter the acoustical performance of the chamber. Vibration and dimensional changes of internal chambers can be avoided by employing a thicker gauge metal. However, the use of thicker metal imposes substantial cost and weight penalties on the muffler. Conversely, there can be substantial cost and weight advantages to be attained by employing lighter gauge metal.
As noted above, the external shell of the muffler shown in copending application Ser. No. 577,495 defines a single large chamber. Mufflers of this design offer several manufacturing advantages in that the deep draws required for plural chamber shells can be avoided, thereby utilizing less metal for the muffler. However, larger chambers may be more prone to shell ring, and in many instances, the prior art patterns of grooves depicted in the above described U.S. Pat. No. 4,924,968 are not sufficiently successful in attenuating noise.
In view of the above, it is an object of the subject invention to provide a stamp formed muffler with chambers that are highly resistant to vibration and associated shell ring.
It is another object of the subject invention to provide a stamp formed muffler with chambers that are dimensionally stable when subjected to forces imposed by flowing exhaust gas.
Another object of the subject invention is to provide a stamp formed muffler that can provide superior reinforcement with lighter gauge metal.
A further object of the subject invention is to provide a stamp formed muffler with internally and externally disposed chamber wall reinforcements.